In decade 1980-1990, concern about the potential threats posed by the warming of Earth led the United States Congress to pass the Global Change Research Act (GCRA). The new law established the United States Global Change Research Program (USGCRP) to coordinate the work of scientists from seventeen federal agencies as they monitored global climate and report to Congress on a regular basis so that Congress could create research-based policy to protect the American people. (USGCRP, 2017)

The first of these National Climate Assessments appeared in 2000, followed by the second in 2009, the third in 2014, and, most recently, the fourth in November of this year.

Taken together, the reports document a vast collection of research findings that affirm that human activity in the form of burning of fossil fuels on a colossal scale has warmed Earth's atmosphere so that now in 2018 the Earth is warmer than it has been over the past 1.2 million years, disrupting the climate that has supported the development of agriculture, settled communities, and ultimately socially and technologically complex entity we call the modern world. (Steffen et. al., 2018)

In the nineteenth century, scientists were interested in heat, what it was, how it worked, how it could be described scientifically. This led inevitable to the sun, the source of heat on earth, so that energy connection between the earth and sun was the place to begin understanding heat.

Our basic understanding of this relationship and its role in global warming can be attributed to the work, among many others, of three men, a French mathematician, physicist and engineer, and Anglo-Irish physicist, and a Swedish chemist. ​

In 1824 Joseph Fourier, the French mathematician, physicist and engineer, developed a mathematical model for earth's heat relationship with the sun.

As the Earth rotates, it receives energy from the sun that warms the side facing the sun. As the warm surface rotates away from the sun, some of that energy radiates back into space.

Fourier found when he did some calculations that Earth was warmer than it should be, given its distance from the Sun and its relatively small size. His calculations showed that the balance between heat gain and heat loss should be well below zero, or at least much, much colder than it was. Fourier realized that there must be some mysterious variable in the atmosphere that blocks the radiation of earth's heat back into space.

About forty years later that mysterious variable would be uncovered by the Anglo-Irish physicist John Tyndall. In 1861 Tyndall was awarded the Royal Society's Bakerian Medal for this discovery. He described what he had discovered to a Royal Society audience on February 7, 1861.

His investigation began with his curiosity about "the transmission of solar and terrestrial heat through the earth's atmosphere..." His curiosity led him to "make the mutual action of radiant heat and gases of all kinds the subject of an experimental inquiry."

In order to test the presence of various gases in the atmosphere, Tyndall constructed an ingenious device that consisted of a 4-foot long, 2.4-inch diameter tube sealed at both ends and supplied with inlet and outlet valves so that different gases could be added and removed.

At one end Tyndall located a heat source and at the other, a target, a thermo-electric pile (thermocouple) attached to a galvanometer that would measure the heat energy passing through whatever gas was being tested. . Tyndall had calibrated the device so that when the tube contained a vacuum, the heat source would register the same temperature at the source and the target. Thus, any gas that blocked (absorbed) the heat energy could be measured in terms of the difference between the heat energy that passed through the vacuum.

He found that oxygen and nitrogen, the principal atmospheric gases were "transparent" and allowed the heat from the source to pass freely through the tube.

However, gases like methane, water vapor, and carbon dioxide blocked; that is, absorbed heat energy.

He had answered the question that Fourier had asked: what is the factor that makes earth warmer than it "should" be? The gases that compose the atmosphere, oxygen and nitrogen are transparent to radiated heat but other gases like water vapor, carbon dioxide, and methane absorb radiated heat, trapping it and adding the mysterious blanketing effect Fourier had observed.

If earth's atmosphere was composed of only oxygen and nitrogen, its temperature, as Fourier had calculated would be much colder than it is. The presence of heat trapping gases: water vapor, carbon dioxide and methane inhibit the amount of radiant heat that escapes into space. Increasing the amount of carbon dioxide will increase the temperature of the atmosphere. Tyndall clearly saw the implications of his findings: "Now if, as the above experiments indicate, the chief influence be exercised by the aqueous vapor [water vapor], every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid diffused through the air; while an almost appreciable admixture of any of the hydrocarbon vapours would produce great effects on the terrestrial rays and produce corresponding changes of climate." (Tyndall, 1861, p. 28)

While Tyndall connected heat absorbing gases in the atmosphere with temperature, our third scientist, the Swedish chemist (recipient of the 1903 Nobel Prize in Chemistry) Svante Arrhenius was the first to use Fourier and Tyndall's ideas to create real-world models of climate. In an 1896 article his research question was: "Is the mean temperature on the ground in any way influenced by presence of heat-absorbing gases in the atmosphere?" In the article he calculated how different concentrations of carbon dioxide might influence the climate, both warming and cooling. (Arrhenius, 1896)

Later, Arrhenius estimated that doubling the amount of carbon dioxide in the atmosphere between 1900 and 1999 would raise the earth's temperature and would make earth into a "hothouse." (NASA)

This brief journey into the past two centuries of scientific investigation shows that heat and climate have long been a matter of intense scientific interest, beginning with Fourier in 1824.

Tyndall demonstrated how even small amounts of so-called greenhouse gases can alter earth's climate.

In the 1980s the potential for crisis was recognized with the passage of the Global Change Act in 1990. Unfortunately, just as with the discovery of the health risks of tobacco when tobacco companies campaigned to undermine the public's trust in science, in the 1990s fossil fuel producers and their political allies sowed confusion and doubt leading to inaction.

Now a century and a half after Tyndall, and with billions of tons of carbon now in the atmosphere, the world now faces "the extinction of the world's tropical reefs, sea-level rise of several meters and the abandonment of the Persian Gulf...James Hansen has called two-degree warming 'a prescription for long-term disaster.' Long-term disaster is now the best-case scenario...[loss of] forests in the ARctic and loss of most coastal cities..." (Rich, 2018)

And from the NCA itself: "While Americans are responding in ways that can bolster resiliane and improve livelihoods, neither global efforts to mitigate the causes of climate change nor regional efforts to adapt to the impacts currently approach the scales needed to avoid substantial damages to the U.S. economy, environment, and human health and well-being over the coming decades." (NCA)